1. Field of the Invention
The present invention relates to a resonant frequency characteristic tag, which is readily capable of dielectric breakdown of an electrically insulating thin film as a dielectric in a high output electric field to short-circuit electrodes on the both sides of the dielectric to destroy preliminarily provided resonant frequency characteristics, and a method of manufacturing the same.
2. Description of the Prior Art
In department stores, supermarkets and general retail shops, various commodity protection means are provided to safeguard commodities against shop-lifting or like theft. To confirm commodities present in a control area as such in an electric manner with electric sensors, each commodity is provided with a resonant frequency characteristic tag, which is made of a flexible material and is provided with a printed circuit having certain particular resonant frequency characteristics. The resonant frequency characteristic tag is applied by means of adhesive to the commodity, or it is suspended therefrom with a string-like member which can not be readily broken. The resonant frequency characteristic tag is designed such that it causes a sensor to produce an alarm sound or the like when it is resonant to a wave at a certain frequency generated from a sensor in an area controlled thereby. The alarm sound permits confirmation of the pertinent commodity and can protect it from theft. However, it is necessary that after purchase of the commodity the tag no longer senses the resonant frequency transmitted from the sensor in the controlled area. Accordingly, after the purchase the printed circuit has been destroyed by "breaking apart" the tag. Alternatively, a metal foil, for instance an aluminum foil, has been applied as an "electromagnetic shield" to the circuit surface of the tag. As a further alternative, the dielectric layer has been made very thin so that exposure of the tag to a high output electric field readily causes short-circuiting of two electrodes of resonant frequency circuit provided on opposite surfaces of the tag, and hence vanishment of the resonant frequency characteristics.
However, where the vanishment of the resonant frequency characteristic is caused unnaturally by, for example, destroying the printed circuit by breaking apart the tag or applying an aluminum foil or like metal foil to the tag, a great deal of man-hour is required for the operation to cause vanishment for each commodity. These unnatural means of causing vanishment, therefore, are not suited for causing vanishment of the resonant frequency characteristics of a great number of tags.
The method of causing ready short-circuit of the printed circuit electrodes by mechanically reducing the thin film of dielectric has a different problem. In this case, the printed circuit is squeezed together with the material of the tag in the thickness direction thereof. Therefore, unless the individual components of the tag have fixed thickness, dielectric breakdown fails to occur in cases when the thickness of the dielectric film is increased.